Wood-waste fuel burners, sometimes known as hog fuel burners, have generally been inefficient in combustion, discharging undesirable amounts of gaseous and particulate pollution. In addition, when the gaseous effluent from such burners is used to heat a boiler, the gases emitted to the boiler have typically been dirty, causing depositions on the heat transfer tubes of the boiler, which, as a consequence, require frequent and expensive cleaning. Some of the disadvantages of wood-waste fuel burners have been addressed in the development of previously known two-stage wood-waste gasifier burners, such as described in U.S. Pat. No. 4,177,740 to Lamb, et al. Such two-stage gasifier burners combust wood waste that is heaped on a grate in the first stage of the burner to form a conical pile of fuel, commonly fed into the burner from below, with preheated underfire air percolating up through the pile in controlled amounts, drying and gasifying the waste fuel in the pile. The volatile gases driven off the pile are then partially oxidized by additional combustion air introduced into the first stage with the total amount of combustion air admitted to the first stage being maintained at less than stoichiometric proportions so that the temperature in the first stage remains lower than that necessary to melt the natural ash, dirt or other inorganic substances in the fuel. The volatile gases are discharged from the throat at the top of the first stage of the burner and enter the second stage, generally located directly above the first stage. In the second stage, secondary combustion air is introduced in an amount sufficient to allow complete combustion of the remaining volatile gases before the products of combustion exit the second stage of the burner to be used for process heat or directly exhausted to the environment.
However, even though prior combustion chamber designs have addressed the problem of gaseous combustion and particulate removal from the gaseous effluent, two-stage wood-waste gasifier burners known in the prior art, as described above, suffer from inefficiencies in operation due in large part to inefficient or impractical fuel supply and ash removal systems. Such inefficiencies in the material handling systems of known gasifier burners tend to cause less efficient gasification of fuel due to uneven distribution of wood-waste on grate surfaces, potentially lengthy and frequent shutdowns for removal of ash and residues due to inefficient ash collection and removal equipment, and overall reduction in wood-waste throughput and thermal generation.
Wet waste other than wood waste can also be burned, but biomass waste of any sort does not burn efficiently at moisture content levels above about 60%. Furthermore, some types of biomass waste, such as sewage sludge, are not easily conveyed if their moisture content falls appreciably below 60%. Below the 60% moisture content level, the sludge tends to become cohesive, requiring unacceptably high consumption of energy for its conveyance. The reason for this is understood to be the tendency of water to combine hygroscopically with fiber in the sludge. At higher moisture content, the sludge moves relatively freely in auger-type conveyors, and any binding within the sludge that occurs tends to be tolerable. These two characteristics of wet biomass waste create the conundrum that if the waste can be efficiently burned, it cannot be efficiently conveyed, and vice versa. A technique is needed that will overcome or at least mitigate the foregoing problem.